1. Field of the Invention
The invention relates to a fiber-optic sensor for measuring a predetermined directional component of an electric field, this sensor comprising a piezoelectronic body, a glass fiber which is rigidly connected to the piezoelectric body in a given link section, and means for detecting a change in the length of the glass fiber.
2. Discussion of Background
Electro-optical effects such as the Pockets effect or the Kerr effect are available for measuring electric field strengths. In this connection, the effect can be utilised that the index of refraction changes linearly (Pockels effect) or quadratically (Kerr effect) with an applied field strength in certain materials. If these materials also have particular characteristics of symmetry, a predetermined directional component of an electric field can also be measured. Whilst so called Pockels sensors require expensive monocrystalline materials of high optical quality which are in many cases sensitive to environmental influences (for example moisture), so called Kerr sensors are mainly suitable for high field strengths because of their low sensitivity. These disadvantages can be circumvented if the electric field strength is measured by means of the inverse piezoeffect. For this purpose, a dimensional change of a piezoelectric body in the electric field is transferred to a change in the length of a glass fiber which can be measured by interferometric means.
In this connection, reference is made to the publication "Fiber-optic Fabry-Perot interferometer and its Sensor Application" T. Yoshino et al. IEEE J. of Quant. Electr. QU-18 1624 (1982). In this publication it is proposed to wind a fiber-optic Fabry-Perot interferometer around a disk of a piezoelectric ceramic, called PZT, and to apply an alternating voltage to the disk. The periodic change in dimension of the PZT leads to periodic intensity fluctuations of the light coupled in the fiber-optic Fabry-Perot interferometer.
Another printed document, "Electric Field Sensitive Optical Fibre Using Piezoelectric Polymer Coating" L. J. Donalds et. al. Electr. Lett 18, 327 (1982), discloses a measuring arrangement in which a glass fiber covered by a piezoelectric polymer and a Mach-Zehnder interferometer are used for determining an electric field strength.
All known piezoelectric sensors measure the amount of the field strength when the direction of the electric field is known. However, they cannot be used if both the amount and the direction of the electric field are to be determined.